Hitherto, polyamide 6,6 resins have been used for relay blocks provided in an engine room of automobiles, but polyamide 6,6 resins have the problem of an increase in dimensional change due to absorption of water, and recently polyamide/polyphenylene ether alloys have been gradually substituted for the polyamide 6,6 resins. For example, JP-A-6-136256 and JP-A-6-141443 disclose relay boxes for automobiles comprising a composition which is composed of polyamide 6,6, polyamide 6, a modified polyphenylene ether, a montanic acid metal salt and (polypropylene) and which is excellent in moldability, mechanical strengths, heat resistance, weathering resistance and recycling properties. Furthermore, JP-A-6-184398 discloses relay boxes for automobiles comprising a composition which is composed of polyamide 6,6, polyamide 6, a modified polyphenylene ether, an A-B-A type block copolymer, a polypropylene, an ethylene-α-olefin copolymer and a montanic acid metal salt and which is excellent in flowability, releasability, mechanical strengths, heat resistance, weathering resistance and recycling properties.
Recently, most automobile parts are electronically controlled, and the role of relay blocks which assemble electrical equipment has become very important. Nevertheless, at present, the space permitted for relay blocks in a highly densified engine room becomes smaller and smaller. Thus, for disposing the relay block in a limited space, the structure of a relay block is very complicated and simultaneously the molded product has many thin wall portions. As a result, it is required that the resin have higher flowability in a mold during the step of producing relay blocks.
For improving the flowability of resin in a mold, there are mainly two methods of reducing the melt viscosity of the resin per se and reducing the solidification speed of the resin. However, in the case of reducing the rate of solidification, the after-shrinkage of molded products becomes great, which causes problems in a use such as relay blocks which are exposed to various environments after molding.
Therefore, it is realized that in a use as a relay block, reducing the melt viscosity of resin per se is the most effective means for improving flowability of the resin in a mold. Accordingly, one of the characteristics required for recent relay blocks is the reduction of melt viscosity of the resin per se constituting the relay blocks.
On the other hand, in relay blocks having a complicated structure, if the rate of dimensional change caused by water absorption is great, the size of the socket portion changes and the socket cannot be drawn out or the socket falls off. Therefore, another characteristic required for recent relay blocks is use of materials which are small in dimensional change caused by water absorption of the materials constituting the relay blocks.
Furthermore, in the case of relay blocks including both thin wall portions and thick wall portions, melt wrinkles (a plurality of wrinkled asperities present in parallel) partially occur on single surface of the molded articles. In Japanese, this phenomenon is called “YUJIWA” by one skilled in the art, and often occur on the part in which the portions greatly differ in wall thickness. This causes not only a problem in appearance, but also is apt to become a starting point of cracking caused by vibration fatigue during operation for a long time, if the melt wrinkles are present on the stress generation portion, and, in the worst case, results in breakage. Thus, improvement in this respect is required. Therefore, an additional characteristic required for recent relay blocks is that the molded relay block has no melt wrinkles on the surface.
The above-mentioned relay blocks according to conventional technologies cannot be said to sufficiently possess the characteristics required for recent relay blocks and the development of relay blocks according to new technologies has been desired.